The nature of the normal response to orally administered compounds is one of tolerance or an active nonresponse to Ags. While there are many ways to achieve this tolerant state, several diseases appear to result when these mechanisms fail. Food allergy is one such manifestation resulting from the failure of normal tolerance to occur. Two phases of this response exist: the first where sensitization to the Ag occurs (with the induction of IgE) and second when re-exposure results in the symptoms of the allergic response. Since the second event occurs quite rapidly the defects in tolerance that exist in food allergy likely occur at the sensitizing phase of the response. The Ag itself also plays a role. Of all the Ags ingested orally only a very small number account for food allergy. We therefore ask what makes an Ag a food allergen? The same foods processed differently can either induce or protect from food allergy (dry roasted vs boiled peanuts) but are these processes are more important for sensitization or the actual allergic response. In the last granting period we looked at distinct milk Ags for their ability to sensitize vs. trigger an anaphylactic response in a murine model of milk induced anaphylaxis. Soluble proteins (ALA, BLG) are largely taken up by small intestinal absorptive epithelium. Casein, which exists in a micellular (aggregated) form is a potent inducer of an IgE response and is taken up by M cells overlying PPs. Interestingly aggregation of ALA and BLG (via pasteurization) alters the pathway of uptake (from I EC to M cell) and this change is associated with an increase in immunogenicity and IgE induction but a decreased ability to trigger the actual anaphylactic response. We have proposed a model whereby sensitization to food allergens occurs via Ag sampling in PPs whereas the actual triggering of anaphylaxis favors soluble Ags that undergo transepithelial transport. We have developed a unique intestinal loop model, where loops containing either PPs or lECs only (without interdigitating DCs) are fashioned. We have shown that tolerance can be induced via peptide (or protein fragment) administration into either loop and that low dose (regulatory) tolerance can be induced with peptides specific for either CD4 or CD8+ T cells (OVA323-339 vs SIINFEKL respectively). In this next granting period we propose to analyze the components of tolerance vs induction of an allergic response by both clarifying the role of the Ag and the nature of the immune response generated. These studies will be performed in concert with studies in Project 1 and 2 (the effect and trafficking of baked milk proteins and IEC transepithelial transport via CD23). We will accomplish these goals by Aim #1. Determine whether the route of sensitization plays a critical role in the subsequent development of an allergic response to milk allergens. Aim #2 Define the individual factors that make Ags food allergens and determine the initial steps in the priming process of milk allergens Aim #3. Determine the role of CDS vs CD4+ T cells in priming vs tolerance.